1. Field of the Invention
The present invention relates to an endoscope which consists of an objective optical system having favorably corrected aberrations, a plurality of image transmitting optical systems and so on.
2. Description of the Prior Art
Due to the restrictions imposed on an outside diameter and a number of lens elements of an objective optical system for endoscopes, such an optical system is configured as a telecentric type which consists, in order from the object side, of a first negative lens unit, an aperture stop and a second positive lens unit. That is to say, most of objective optical systems for endoscopes have a composition illustrated in FIG. 1 and satisfy the sine condition shown below: EQU I=f.multidot.sin .theta.
wherein the reference symbol I represents 1/2 of a film size, or an image height, the reference symbol f designates a focal length of an objective optical system and the reference symbol .theta. denotes a half of a field angle.
An objective optical system satisfying the sine condition mentioned above produces distortion which is abruptly aggravated as a field angle of the optical system is enlarged. Relationship between distortion DT(.theta.) produced by the objective optical system and the angle .theta. thereof is expressed as follows: EQU DT(.theta.)=(cos .theta.-1).times.100 (%)
When a size of an image deformed by distortion is represented by y and a size of an ideal image which is calculated according to the paraxial theory is designated by y.sub.0, DT is given by the following formula: EQU DT=(y-y.sub.o)/y.sub.o .times.100 (%)
This distortion is visualized as illustrated in FIG. 3A through FIG. 3F.
The objective optical system for endoscopes which satisfies the sine condition produces negative distortion (barrel type distortion) which is abruptly aggravated as the field angle 2.theta. is enlarged, for example, in a relation listed in the table shown below:
______________________________________ Field 2 .theta. 40.degree. 60.degree. 80.degree. 100.degree. 120.degree. 140.degree. angle Dis- DT (.theta.) -6 -13.5 -23 -36 -50 -66 (%) tortion ______________________________________
FIG. 3A through FIG. 3F show views illustrating how images are actually deformed in appearances thereof by distortion. Illustrated in these drawings are images of lattice patterns in air consisting of vertical and horizontal lines arranged at equal intervals on a plane perpendicular to an optical axis which are formed by an objective optical system producing distortion DT(.theta.) of 30% at a maximum image height.
The conventional objective optical system for endoscopes have, as is understood from the foregoing description, wide field angles, the telecentric type compositions, favorably corrected aberrations, and compact sizes, satisfy the sine condition, and produce remarkable negative distortion.
On images formed by the objective optical systems for endoscopes which produce the negative distortion as described above, marginal portions are small and deformed as compared with central portions. For this reason, these objective optical systems do not permit accurate measurements and analyses of shapes and forms when the optical systems are used, for example in the industrial field, for inspections and observations of objects. Further, these objective optical systems may be causes of erroneous diagnoses when used in the medical field.
As conventional examples of objective optical systems for endoscopes which are configured so as to favorably correct the negative distortion, there are known, for example, the optical systems disclosed by Japanese Patent Kokai Publication No. Hei 3-39,915 and Japanese Patent Kokai Publication No. Hei 4-146,405. These objective optical systems have compositions which are illustrated in FIG. 1 and FIG. 2 respectively. Each of these objective optical systems is of the retrofocus type which consists of a negative front unit F and a positive rear unit R disposed on both sides of an imaginary stop S, and uses an aspherical surface ASP in the front unit F or the rear unit R for correcting distortion.
A portion of an endoscope which is to be inserted into objects to be inspected has a diameter which is restricted dependently on inside diameters and inlet ports of the objects to be inspected. Since objective optical systems for endoscopes are to be disposed in the portion of endoscopes to be inserted into the objects to be inspected, diameters of the objective optical systems are restricted and the objective optical systems are composed generally of lens elements having diameters of 6 mm or smaller. On the other hand, lens elements which have aspherical surfaces are formed by polishing or heating glass materials to high temperature for fusion and molding these glass materials. Regardless of the process to form aspherical lens elements, there is posed a common problem that the aspherical lens elements cannot have design surface precision due to eccentricity caused by positioning errors, ununiform polishing, inadequate molding conditions and so on.
Allowances for eccentricities and surface precision of aspherical lens elements having small outside diameters cannot be loose and allowances which are equal to those for aspherical lens elements or stricter allowances are required for the aspherical lens elements having the small outside diameters. Accordingly, manufacturing becomes more difficult, eccentricity is to be made more easily and surface precision is apt to be lower as aspherical lens elements have smaller outside diameters. Further, allowances for injuries on lens surfaces and adhesion of foreign matters to lens surfaces are stricter for lens elements having small outside diameters than those for lens elements having large outside diameters.
As is understood from the foregoing description, manufacturing of the conventional endoscopes systems which use the aspherical surfaces adopt the aspherical lens elements is difficult and therefore requires a high cost.
In the conventional objective optical system illustrated in FIG. 1, an aspherical surface is used on a meniscus lens element which is disposed on the object side and has a convex surface on the object side. This meniscus lens element has a very strong negative power for obtaining a wide field angle which is indispensable for an objective optical system for endosocpes. For obtaining the very strong negative power of the meniscus lens element, a glass material which has a refractive index as high as possible is selected so that an image side surface of the meniscus lens element will not have a small radius of curvature and the meniscus lens element will be formed easily. Further, the glass material selected for this meniscus lens element has low dispersing power since an offaxial principal ray is high on the meniscus lens element and this lens element produces remarkable chromatic aberration.
It is therefore desirable to use a glass material having a high refractive index and a low dispersing power for a negative meniscus lens element disposed on the object side as in the conventional example of the objective optical system illustrated in FIG. 2. However, the glass material which has such a high refractive index and such low dispersing power has a fusion point on the order of 600.degree. to 700.degree. C. When the aspherical lens element is to be manufactured by molding as described above, it is improper to select the glass material which has the high refractive index and the low dispersing power.
For this reason, a glass material which has a fusion point on the order of 400.degree. to 500.degree. C., a high refractive index and a high dispersing power is selected for the aspherical lens element in the conventional objective optical system illustrated in FIG. 1. As a result, this objective optical system allows remarkable lateral chromatic aberration to be produced by the meniscus lens element which is disposed on the object side. For correcting this chromatic aberration, the objective optical system uses a cemented lens component in the rear unit thereof, requires proper selection of glass materials for lens elements composing the cemented lens component and adopts a small radius of curvature on the cemented surface. As a result, the cemented lens component requires a high manufacturing cost.
For the reasons described above, it is undesirable to use aspherical surfaces in objective optical systems for endoscopes. In addition, when objective optical systems have wide field angles, departures from reference spheres become large, marginal portions of aspherical surfaces have shorter radii of curvature than those of central portions, thereby making it difficult to design and manufacture compact objective optical systems.